Directional couplers are essential RF circuits to measure reflected and transmitted RF power. FIG. 1A illustrates a representation of a basic configuration of a directional coupler provided by a single-section coupled line. The coupled line is a result of two transmission lines in proximity to each other; a main line and a coupled line. As shown in FIG. 1A, a pair of coupled lines creates a 4-port device with two planes of reflection symmetry. Following an ideal directional coupled-line coupler and an input at P1, P2 is a through port, P4 is isolated, and P3 is coupled and outputs a signal having 90° phase shift from the signal at the input port P1 (via backward coupling). The single-section coupler is typically implemented in coaxial cable or as a stripline or microstrip to provide a quarter-wavelength (λ/4) directional coupler.
The assignments of these ports may change for a directional coupler depending on the port used as input and, in some cases, whether the directional coupler is a forward wave coupler or a backward wave coupler.
FIG. 1B illustrates a representation of a basic configuration of a quadrature hybrid directional coupler (e.g., a branch line coupler) where λ/4 RF transmission lines are used to provide the phase change. Referring to FIG. 1B, an input provided at P1 is isolated at P4 and output with negative phase change at output ports P2 and P3.
FIG. 1C shows a lumped element backward directional coupler, which is based on the conventional parallel coupled line directional coupler. As shown in FIG. 1C a lumped element approach can accomplish similar coupling as the transmission line approach while providing the scalability needed to permit the directional coupler to be used in applications for RFICs (radio frequency integrated circuits) and MMICs (monolithic microwave integrated circuits).